A lot. The last release was build 1465 and now it sits at 2385. I'm working on the important "modification" engine that allows me to easily make a wide range of abilities and adjustments to the game. Anything that "easily" does anything takes a lot of work to implement. But it's coming along. I'm cleaning things and will release it in the next few days. 

When it is done, the game will have new abilities and towers. (Not many, but the foundation will be laid for quickly making more.) Level editors can specify loadouts for their levels. And truly the game will sit from a place resembling the final product in terms of what you do with the game besides play it.

Anyway, keep the faith. I'm working every day on ATT.